New Trigger: is mergable state

[blake-newman]

Currently it is not possible to trigger an action based on the mergable state of the PR; this creates a limitation on some possible workflows.

An example workflow would be auto updating PR when out of sync with the base branch. See this issue (https://github.com/pascalgn/automerge-action/issues/16#issuecomment-546625489) for more context on functionality desired.

Unfortunately there is no current way to easily implement the above solution via GitHub actions.

[AndreaGriffiths11]

Thanks for the feedback! I've taken your suggestion and passed it along to the appropriate teams. Thanks again for reaching out 😀